Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols
The hydroperoxyl radical (HO 2) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO 2 concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO 2 uptake by aerosol involves conversion to H 2O 2, but this is of limited efficacy as an HO 2 sink because H 2O 2 can photolyze to regenerate OH and from there HO 2. Joint atmospheric observations of HO 2 and H 2O 2 suggest that HO 2 uptake by aerosols may in fact not produce H 2O 2. Here we propose a catalytic mechanism involving coupling of the transition metal ions Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO 2 to H 2O in aqueous aerosols. The implied HO 2 uptake and conversion to H 2O significantly affects global model predictions of tropospheric OH, ozone, carbon monoxide (CO) and other species, improving comparisons to observations in the GEOS-Chem model. It represents a previously unrecognized positive radiative forcing of aerosols through the effects on the chemical budgets of major greenhouse gases including methane and hydrofluorocarbons (HFCs).